One thing I hardly ever do is discuss what most people would consider real "Evolutionary Psychiatry." That is, how do diseases such as schizophrenia or autism, which in their worst forms are obviously so detrimental to evolutionary fitness that they would seem to represent a genetic dead end, continue in the gene pool. It doesn't make much sense at first glance. However, one could postulate that, just as the heterozygote carriers of sickle cell anemia are relatively protected against malaria, having some schizophrenia-risk genes could convey some sort of benefit for close relatives. And one must also consider the possibility that the schizophrenia phenotype is worse now than it may have been for much of human history - with plenty of vitamin D, no wheat (speculatively :-) ) or common modern pathogens, it is possible the schizophrenia may not have developed as fully or been as debilitating.
Given dopamine's role in creativity, motivation, and drive, the suspected genetic advantage of being a relative of a schizophrenic is that you may have a bit of extra dopamine, but not so much it will make you psychotic. Psychotic thought is disjointed and disorganized - creative thought is taking seemingly unrelated or unexpected ideas and bringing them together in a novel way.
Sounds reasonable. But what about the data proving it? Well, there has been a lot of speculation looking back at known geniuses and their psychopathologies. It is felt it is no coincidence that many geniuses were not particularly psychologically healthy. A more recent study selected 30 creative writers at a workshop and compared them to controls - writers had higher rates of affective disorders (several variations of this study have been done with the same results). Studies of bipolar individuals showed they scored higher on scales measuring creativity than folks with unipolar depression or non-creative controls - the bipolar folks scored the same as creative healthy controls.
In Iceland, the histories of 486 male relatives of schizophrenics were investigated - these men were more likely to be prominent historically than the general population, and there was a significant increase in those who were specifically successful in creative endeavors.
But all those studies are small, and many rely on historical records. However, a brand new paper from the British Journal of Psychiatry documents a large, population based study of 300,000 individuals with severe forms of affective disorders or schizophrenia from a large population registry in Sweden, where there is data on hospital admissions, diagnoses, IQ, occupation, and detailed family records as well. The were able to find several tens of thousands of folks with bipolar disorder and schizophrenia, and over two hundred thousand diagnosed with unipolar depression.
The results? People with schizophrenia and bipolar disorder (with the effect stronger in schizophrenia) were more likely to have parents and siblings who were in creative professions. Bipolar patients also were more likely to have creative offspring. The ORs aren't huge - ranging from around 1.2 to 1.6, but the bars don't cross the 1.0 line suggesting a real correlation. There were no strong statistically significant correlations between having a relative with unipolar depression and engaging in creative professions (described as "including scientific and artistic occupations.") As one would expect for a genetic link, as relationships got further away (half-siblings, cousins, etc.) the correlations weakened accordingly.
The reverse sort of "non-creative" correlation was also true - folks with schizophrenia were significantly less likely to have relatives who were accountants and auditors.
And the IQ connection (only measured in men in this Swedish registry) - those in creative professions had a higher IQ on average, however, the IQs of people with schizophrenia, unipolar depression, bipolar depression and their relatives were lower on average than people without any of the three diagnoses. IQ was accounted for in the correlations we talked about in the previous paragraphs and did not weaken the genetic association between creativity and severe psychiatric illness (specifically bipolar disorder and schizophrenia).
Well. That is all very interesting! I might go on to be a real Evolutionary Psychiatrist after all.